CORE LAB Only - This study will evaluate the efficacy of in vitro fertilization on unstimulated cycles in 20 women with pure tubal factor infertility, undergoing a maximum of 3 natural cycles of in vitro fertilization. This study seeks to examine 1) the efficacy with which IVF can be applied to natural cycles, and 2) to define the parameters which will enhance or diminish success. All samples have been collected and the GCRC is assisting with retroanalysis of stored samples.